Botrytis blight, or
gray mold, as it is commonly known, has an exceptionally wide host range with
well over 200 reported hosts. The fungus can occur as both a parasite and a saprophyte
on the same wide range of hosts. This fungus disease is intriguing in that it
can cause a variety of plant diseases including damping-off and blights of flowers,
fruits, stems, and foliage of many vegetables and ornamentals. It is a major cause
of postharvest rot of perishable plant produce, including tomatoes at harvest
and in storage. The disease can occur both in the greenhouse and in the field.
Besides tomato, gray mold is of concern to other vegetables including snap and
lima beans, cabbage, lettuce and endive, muskmelon, pea, pepper, and potato.

Cause

Gray
mold is caused by the fungus Botrytis cinerea. One-celled spores are borne
on branched conidiophores, and the arrangement of the spores gives the fungus
its name, from the Greek botrys, meaning a bunch of grapes. Use of a hand
lens may reveal the characteristic grape-cluster arrangement of spores. As the
conidiophores dry out, they gently move and liberate the spores: usually air movements
are sufficient to get the spores airborne. The fungus often establishes itself
on injured tissues and can persist as a saprophyte for long periods. Upon occasion,
black sclerotia of variable size form on or just below, the host surface. The
sclerotia have a black rind and a light interior composed of a dense mass of hyphae,
or threads, of the fungus. Sclerotia measure up to 3 mm (occasionally 5 mm) in
length and are usually smaller and thinner than those of the white mold fungus
Sclerotinia sclerotiorum. The sclerotia germinate to produce conidiophores
or, rarely, give rise to small cup-shaped structures (apothecia), which are the
sexual stage of the fungus. Sclerotia are resistant to environmental extremes
and act as overwintering resting bodies.

Symptoms

Stem
lesions on seedling tomatoes can occur at, or just below, the soil level. Entry
to the stem may occur through senescent cotyledons or damaged tissue. Stem lesions
can also occur later during the growth of the crop. Stems can become infected
through leaf scars, dead leaves, or any form of stem damage. Stem lesions often
partially girdle the stem, but sometimes the whole stem is affected and the plant
is killed. Petiole lesions appear very similar to those on the stem and often
result from infection and colonization of a leaflet. Leaflet lesions often start
from senescent tissue or any physical or chemical damage. The pathogen can grow
along a petiole to the main stem and can eventually form a lesion there. Flower
parts that have fallen onto leaves are a common starting point for leaflet colonization.
Pollen from flowers and the flower parts can act as a stimulant to B. cinerea
spores, not only stimulating germination, but also increasing the virulence of
the isolate. In the field the fungus appears as a gray, velvety covering of spores
on dying flowers (fig. 1) and on the calyx
of fruit (fig. 2). Senescent flowers are frequently colonized
by Penicillium spp. (blue mold), and this fungus may be confused with gray
mold. Infections spread from flowers and fruit back toward the stem; the stem
turns beige to white and develops a canker, which can girdle the entire flower
hand (fig. 1). Immature green fruit turn light brown
or white, starting at the point where they touch other infected plant parts. A
soft rot may develop with the fruit skin remaining intact, but the inner tissue
becomes mushy and watery (fig. 3). Later, a gray fuzzy mold
develops (fig. 4), and sclerotia may appear (fig.
3, note under dead calyx or sepals). If this stage occurs in the greenhouse,
the floor of the house will be littered with fruit that have fallen off the plant.
In the field, the alleyways will be filled with discarded fruit (fig.
5). Green fruit can also become infected directly by airborne spores instead
of by contact with other infections. White circular (halo) spots appear on the
fruit and have been termed "ghost spots." These spots will persist and can appear
on green, breaker, and mature fruit (fig. 6). As fruit ripens,
the color of the halos changes from white to yellow. The "ghost-spot" symptom
results from spore germination and penetration of the fruit, which is only susceptible
to attack up to cherry size. As soon as the surface of the fruit is shiny, it
is no longer susceptible. Penetration of the mycelium of Botrytis into the fruit
produces a host reaction preventing any further mycelial growth and results in
localization of the pathogen. The halo forms around the point of entry.

Click
on each photo for Magnification

Epidemiology

The
fungus overwinters as sclerotia or as mycelium in plant debris and may be seedborne
as spores or mycelium in a few crops. Other crops may also serve as sources of
the pathogen and are likely to cross-infect. Conidia are airborne and may also
be carried on the surface of splashing rain drops. High relative humidities are
necessary for prolific spore production. In the field, spores landing on tomato
plants germinate and produce an infection when free water from rain, dew, fog,
or irrigation occurs on the plant surface. Optimum temperatures for infection
are between 65° and 75° F (18° and 24° C), and infection can
occur within 5 hours. High temperatures, above 82° F (28° C), suppress
growth and spore production. Dying flowers are a favorable site for infection,
but infections can also result from direct contact with moist infested soil or
plant debris. In the greenhouse, stem lesions develop either by direct colonization
of wounds or through infected leaves. The presence of external nutrients, such
as pollen grains in the infection droplet, can markedly increase infection. The
type of wound is said to influence stem lesion development; breaking off leaves
is reported to give a lower incidence of stem lesions than cutting off leaves
with a knife, leaving a stub.

Control

Cultural.
There is no known resistance to B. cinerea in tomato cultivars. During
a recent epidemic of gray mold in a grower's field in upstate New York where a
variety trial was in place, the following observations were made: the most susceptible
varieties present were Castleking, Mountain Pride, Pik-Red, and Pirate; intermediate
susceptible varieties were Freedom, Revolution, Horizon, and Sunny; lower infections
were noted for FIoraTom, Duke, and Jackpot. It should be noted that these results
are from one year, and environmental conditions favoring gray mold may not occur
every year. In greenhouse operations, effective control can be achieved by preventing
predisposing conditions (high relative humidity and cool temperatures), by adequate
spacing and pruning to promote ventilation, by careful handling to prevent wounding,
and by removing inoculum sources through adequate plant sanitation.

Fungicides.
Most fungicides registered for use on tomato are protective in their action
and will not suppress an established infection. Because of this, treat the crop
before infections build up and especially when cool and humid or wet conditions
are prevalent. Refer to the most recent issue of Cornell Vegetable Recommends
for registered fungicides, rates and methods of application. A special note is
required on the development of fungicide-resistant isolates of Botrytis.
Repeated use of benomyl (Benlate) can result in the development of benomyl-resistant
strains of Botrytis, rendering the material ineffective. Pathogen resistance
is thought to be related to the development of chemicals with sitespecific modes
of action. Many new chemicals vary in their chemical structure, but have similar
modes of action that appear to result in cross-resistance. Thus benomyl-resistant
Botrytis is cross-resistant to other chemicals including thiophanate-methyl,
thiabendazole, and carbendazim. Some evidence suggests that alternating benomyl
with other registered materials may slow the buildup of benomyl-resistant strains.

Brand
names and varieties are mentioned for clarity. No endorsement or discrimination
is intended.

This publication is issued
to further Cooperative Extension work mandated by acts of Congress of May 8 and
June 30, 1914. It was produced with the cooperation of the U.S. Department of
Agriculture, Cornell Cooperative Extension, New York State College of Agriculture
and Life Sciences, New York State College of Human Ecology, and New York State
College of Veterinary Medicine, at Cornell University Cornell Cooperative Extension
offers equal program and employment opportunities. Lucinda A. Noble, Director.